You may know about Mailinator, and how it will hold any email sent to anything-you-want@mailinator.com for a few hours -- great for throw away registrations!

Using Mail.app's RSS reader, you can make Mailinator even more useful by having Mail.app monitor the RSS feed for the Mailinator inbox name you invent. In that way, you don't have to remember to go check periodically for new mail. It's always there!

Go to Mailinator.com and enter some made-up name to create your "inbox."

Click on the RSS link.

Look for the "Subscribe in Mail" link in the lower right.

That's it; you're done. Optionally, in Mail, click the little Up Arrow that appears next to the feed to make the feed appear as another inbox.

I followed your instructions, but when I subscribe to the feed, Mail opens, the feed is created, but a warning sign appears "The feed could not be loaded because the content is not in known feed format."